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How to diagnose suspected catheter related infection

Intravascular catheter related infections with their associated complications have become a significant problem in the hospital setting. 50-85% of primary bloodstream infections in hospitalised patients are considered to be catheter associated. Catheter-related blood stream infections (CRBSI) are the 3rd most common cause of health-care associated infections and the leading cause of infection in critically ill patients.

Types of catheters

  • There are many different types of intravascular catheters (also known as lines) that are inserted into the veins or arteries of patients for a multitude of different reasons e.g. for the administration of fluids, medications, nutrients, blood products and chemotherapy as well as for dialysis.
  • Peripheral intravenous (IV) catheters give medication or fluids into a vein near the skin surface. Similarly, a catheter may be inserted into an artery just under the skin, but these are usually used to measure the patient's blood pressure and to take arterial bloods and not for giving medication or fluids.
  • Single or multi-lumen central venous catheters are inserted into the neck or groin areas. A central venous catheter is threaded into the vein through the skin and along the vein until it reaches a larger vein near the heart.  Central venous catheters can be left in place for a longer period  of time than  peripheral IV lines.
  • Surgically implanted vascular devices sit completely under the skin and access is through the skin into the port where the medication can be released slowly into the circulatory system. This type of subcutaneous venous port device is commonly used for chemotherapy for cancer patients as it can stay in place for many months.
  • Dialysis catheters are used for dialysis in patients with kidney failure.
  • PICC catheters are similar to central venous catheters but are inserted into the arm instead of the neck.
  • Tunnelled catheters are placed under the skin with only a small port that sits outside the skin e.g. Groshong and Hickmann lines. 

≥90%  of catheter-related infections are due  to infections of central  venous catheters.

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Source of catheter infection 

Approximately 50% of catheter-related infections originate from the bacteria that live on the skin which travel through the insertion site along the catheter and form a slimy layer in which the organisms live. In this slime (also known as a “biofilm”) the organisms are sheltered from the immune system and from any antibiotics that may be given. Because of this biofilm, it usually means that ultimately the only way to treat the infection is to remove the catheter.

Infections may also be caused by a contaminated hub or port which may be infected by the hands of the health care workers who touch this hub with unclean hands or gloves  and  then  inject medications, fluids and  blood through this hub,  thereby helping  the organisms to get directly into the bloodstream of the patient. 

Diagnosis

Establishing a diagnosis of a catheter related infection involves both clinical and laboratory components. Generally, the clinical features are unreliable and non- specific including fever, chills, low blood pressure and confusion. Oozing or redness around the site of insertion of the line is a good indication that the catheter is infected. If there is no apparent source of infection or sepsis in a patient who has an intravascular catheter, the possibility of a CRBSI should be considered.

 Laboratory testing should include cultures of both the blood and the catheter if it is removed. An alternative diagnostic method could involve taking a blood culture through the port of the catheter and a blood culture taken from the patient's arm vein or another site (called the peripheral blood culture). If a CRBSI is present, the blood culture that was taken through the line will signal positive in the blood culture machine at least 2 hours before the peripheral blood culture signals positive. Care must be taken that both blood cultures are taken as aseptically as possible i.e. make sure to clean the skin or catheter hub thoroughly with alcohol or chlorhexidine before taking the sample.

 If the line is removed and sent for culture of micro- organisms, a 5cm segment that includes the tip of the line should be sent in a sterile container as soon as possible to the laboratory. In the laboratory two different culture methods are used to assess if the line is colonised with any micro-organisms. 

  • A semi quantitative “Maki roll” catheter segment culture is performed. This involves rolling the catheter over an agar plate at least four times to inoculate any organisms that may be adhering to the outer segment of the catheter. Then the plate is put in the incubator for 48hours, after which the plates are then assessed for the growth of any micro-organisms. The number of bacterial colonies growing  on the plate  is useful for determining the probability of whether those organisms may be causing an infection  or not.

When the catheter is pulled out through the skin, organisms that live normally on the skin may get caught on the line, but these organisms would be present in much smaller numbers than if the organisms had been growing  on the line and  thereby potentially  causing a line infection.  The organisms causing catheter infections are usually the same ones as those growing normally on the skin (also known as the skin's “normal flora”), so it can often be challenging to differentiate between contamination versus colonisation of the catheter.

—   1-4 colonies: signifies possible contamination by the skin's normal flora

—   5-14 colonies: organisms are of uncertain significance

—   ≥15 colonies: suggests that the organisms were growing in a high enough number to be colonising the line and  possibly causing a line- related infection.

Any number of yeast colonies growing on the agar plates is suggestive of a fungal line infection.

  • The second method involves flushing the catheter through with a small amount of nutrient  broth,  plating the broth  onto  an agar  plate  and  then  counting the number of colonies growing  on the agar  plate.                 —   1-9 colonies are equivalent to < 1000 colonies per millilitre of broth and is of doubtful significance.         —   ≥10 colonies are equivalent to >1000 colonies per millilitre of broth and is an indication that colonisation of the catheter lumen is likely. 

Prevention is better than cure 

Touching of the line should be kept to a minimum to avoid potential contamination by health care workers. Proper attention to aseptic techniques when inserting the catheter and meticulous hand washing and  wearing of gloves  before touching the hub or port should help ensure that the incidence of catheter-related infections is kept  as low as possible.

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